1. Field of the Invention
The present invention relates to a method of manufacturing an air bridge type structure for supporting a micro-structure fabricated by means of micro-mechanics, and to a mold substrate suitable for use therein, and to an air bridge type structure and a micro-structure using the same and also a probe for detecting tunneling current or micro-forces, using the same.
2. Related Background Art
In recent years, micro-mechanics technology has given rise to consideration of micro-machines having small movable mechanisms. Particularly, multiple small micro-machine parts can be fabricated on a substrate with high reproducibility, owing to micro-structures being fabricated using semiconductor integrated circuit formation technology (the semiconductor photolithography process). Accordingly, the arraying, and lowering of costs there, have become relatively simple, and responsivity higher than that of conventional machine structures can be expected due to the miniaturization thereof. Surface micro-machining which uses a sacrificial layer is a type of semiconductor photolithography micro-mechanics technology which enables the fabrication of wobble motors, linear micro-actuators, and the like; various devices have been developed using this process.
There are four typical methods for surface micro-machining using a sacrificial layer, as described below.
A first method involves a poly-silicon film, SOI (Si on Insulator) film, or the like to be fabricated into a micro-structure, in which film thin-film formation is performed on a silicon substrate over a silicon dioxide film thereon. This film is patterned into a desired form, following which the silicon dioxide film is removed by means of an aqueous solution of hydrofluoric acid. Linear actuators having a tunneling tip (D. Kobayashi et al., "An Integrated Lateral Tunneling Unit", Proceedings of the IEEE Micro Electro Mechanical Systems Workshop 1992, pp 214-219) and the like can be manufactured with this method. FIGS. 1A to 1E illustrate an overview of this process. First, sequential layering is performed on a substrate 512 of a silicon dioxide film 511 which serves as a sacrificial layer, a poly-silicon film 513 which becomes the structure, and a nickel mask layer 514 (FIG. 1A), following which the poly-silicon film 513 is worked using the nickel mask layer 514 as a mask, thus forming a micro-structure of the poly-silicon film 513 (FIG. 1B). The nickel mask layer 514 is then removed (FIG. 1C), following which the silicon dioxide film 511 is etched by means of an aqueous solution of hydrofluoric acid, and a micro-structure B separated from D by a gap is formed using side etching of the silicon dioxide film underneath the above structure (FIG. 1D). Vapor deposition of a metal film 515 is performed by sequential layering of Cr and Au on the surface of the structure (FIG. 1E), thus fabricating a micro-structure having electroconductivity.
The silicon dioxide film supporting the undersides of the micro-structures A and C shown in FIG. 1D is etched back, so that the structures A and C form an overhang (beam forms) over the substrate D. Accordingly, as shown in FIG. 1E, there is no electrical connection between the structures and the substrate, or between the two structures A and C, even after vapor deposition of the metal film 515 is performed. As such, multi-layered wiring (which would be desirable for simplification of the number of wires upon the substrate) is difficult, thus making miniaturization of the device difficult.
A second method involves forming a micro-structure layer on a silicon substrate and using undercutting by etching the Si substrate underneath the micro-structure so as to cut the micro-structure away from the substrate. Micro-motors (T. Hirano et al., "Dry Releasing of Electroplated Rotational and Overhanging Structures", Proceedings of the IEEE Micro Electro Mechanical Systems Workshop 1993, pp 278-283) and the like can be fabricated using this method, wherein the silicon substrate is used as a sacrificial layer.
With this method, the silicon substrate supporting the micro-structure underneath is etched back, so that the structures form an overhang (beam form) over the substrate. Accordingly, electrical connection between the structures and the substrate, or between the structures is difficult.
A third method involves a process of sequentially layering a sacrificial layer of silicon oxide film and a poly-silicon film which becomes the micro-structure layer. The poly-silicon film is patterned into a desired form, following which the silicon oxide film is removed by means of an aqueous solution of hydrofluoric acid. This fabrication method allows for fabrication of wobble micro-motors (M. Mehregany et al., "Operation of Microfabricated Harmonic and Ordinary Side-Driven motors", Proceedings IEEE Micro Electro Mechanical Systems Workshop 1990, pp 1-8), micro-mirrors (N. C. Tien et al., "Surface-Micromachined Mirrors for Laser Beam Positioning", Proceedings of the 8th Int. Conf. on Solid State Sensors and Actuators, 1995, pp 352-355), micro-optical benches (L. Y. Lin et al., "Micromachined Integrated Optics for Free-Space Interconnections", Proceedings of the IEEE Micro Electro Mechanical Systems Workshop 1995, pp 77-82), and the like.
As an example, FIGS. 2A to 2F illustrate the method of fabrication of a hinge portion for a micro mirror. A silicon nitride film 611 is formed on a substrate 610, and thereupon are conducted the sequential steps of forming a first sacrificial layer 612, a first structure layer 613, a second sacrificial layer 622, and a second structure layer 623, and patterning thereof (FIG. 2A).
Next, a silicon oxide film serving as a third sacrificial layer 632 is formed (FIG. 2B) and patterned, thus forming the third sacrificial layer 632 (FIG. 2C). Then, a poly-silicon film 625 is formed (FIG. 2D) and patterned, thus forming a third structure layer 633 (FIG. 2E). Subsequently, the first, second, and third sacrificial layers are removed by means of etching, thus forming an air bridge type hinge 650 which supports across a gap mirrors 640 connected to a slider 630 (FIG. 2F). Finally, metal film serving as a mirror reflecting film is formed.
Regarding the method of fabricating air bridge type structures such as hinges or the like wherein the above substrate and structures are fixed mechanically, or wherein the structures are fixed one to another mechanically, film formation to the side wall of trench formations can be performed by forming the sacrificial layers and the structure layers by means of CVD (Chemical Vapor Deposition) method, thus facilitating mechanical or electrical connections of the substrate and the structures or of the structures one to another.
However, in -the event that a sacrificial layer and a structure layer is to be formed of the overhanging structure shown in FIGS. 1A to 1E, the sacrificial layer and structure layer also are undesirably formed on the back surface of the beams of the structures A and C, and the structural layer on the back surface of the beams cannot be removed. Accordingly, the device must be designed so that there is no undercutting of the underside of the structure layer. Also, in FIGS. 2A to 2F, heat processing is applied to the first and second structure layers upon formation of the third sacrificial layer and third structure layer. Due to this, there is the possibility that the film stress of the first and second structure layers may change and the structure layers may bow following removal of the sacrificial layer, making film stress monitoring and film stress control accompanying the monitoring crucial.
A fourth method involves forming the structures on a substrate having a sacrificial layer, using X-ray lithography processing and electroplating film formation. This is a method for forming micro-actuators ideal for electromagnetism driving, such as micro-motors (T. Guckel et al., "A First Functional Current Exited Planar Rotational Magnetic Micromotor", Proceedings of the IEEE Micro Electro Mechanical Systems Workshop 1993, pp 7-11) and linear actuators (T. Guckel et al., "Micro Electromagnetic Actuators Based on Deep X-Ray Lithography", Proceedings of the International Symposium on Microsystems, Intelligent Materials and Robots 1995, Sendai, Japan, pp 21-24), and the like.
This method enables the formed micro-structure to have a higher degree of precision and a higher aspect ratio than that of the aforementioned three other methods. However, with this method, electrode formation on the side walls of the structure, and multi-layered wiring comprising electrode wiring between structures is difficult. Accordingly, in the event that an electromagnetic coil is to be formed, methods have been used such as using wire bonding for inter-structure electrical connections or winding the coil beforehand and assembling afterwards. Consequently, it becomes difficult to have a high concentration of wiring, and also, productivity is low.